bone marrow A soft tissue contained within the central cavity and internal spaces of a bone. At birth and in young animals the marrow of all bones is concerned with the formation of blood cells: it contains haemopoietic tissue and is known as red marrow. In mature animals the marrow of the long bones ceases producing blood cells and is replaced by fat, being known as yellow marrow.

bone marrow (marrow) n. the tissue contained within the internal cavities of the bones. At birth, these cavities are filled entirely with blood-forming myeloid tissue (red marrow) but in later life the marrow in the limb bones is replaced by fat (yellow marrow).